PHP Mode for Emacs Code

(php-mode-modified): Use a string literal rather than a computed
value.

(php-tags, php-tags-key): Move definition to above
php-mode to avoid byte compiler warning. Use eval-when-compile
for regexp-opt.
(php-font-lock-keywords-1): Use font-lock-preprocessor-face for
PHP and ASP tags.

(php-mode): Overwrite C-style macros rule to recognize PHP tags
instead.
(php-tags, php-tags-key): New constants.
(php-font-lock-keywords-1): Use php-tags-key for syntax
highlighting of PHP tags.

(php-mode): Add commented-out c-lang-defvar statements below
commented-out c-lang-defconst.

(php-mode): Turn on necessary c-mode features with c-add-language,
like c-symbol-key.

(php-mode-modified): Resolve setting constant by
using format-time-string, but still using eval-when-compile.

ChangeLog entries for recent commits.

Use correct regular expression.

(php-beginning-of-defun-regexp): Support function
keywords like public, private per #1892662.

(php-mode): Remove definition php-mode-syntax-table
and other XEmacs bits that need to be revisited.